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PS-62-84064 



LIBRARY 

RESEARCH REPORTS DIVISION 
NAVAL POSTGRADUATE SCHOC 
MONTEREY, CALIFORNIA 9394( 



NAVAL POSTGRADUATE SCHOOL 

Monterey, California 




HF BROADBAND WHIP ANTENNA 
EVALUATION 

Richard W. Adler 
H. B. Shaw III 

October 1984 



Approved f or pu bli c release; distribution unlimited. 



Prepared for: Naval Electronic Systems Command 

r-.-r>r^^ ^-110-12 

FEDDOCS 
D 208.14/2: 
NPS-62-84064 



ihington, DC 20363 



NAVAL POSTGRADUATE SCHOOL 
Monterey, California 

Commodore R. H. Shumaker D. A. Schrady 

Superintendent Provost 

The work reported herein was supported by the Naval Electronic Systems 
Command . 

Reproduction of all or part of this report is authorized. 
This report was prepared by: 



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1. REPORT NUMBER 

NPS-62-84064 



2. GOVT ACCESSION NO 



3. RECIPIENT'S CATALOG NUMBER 



4. TITLE (and Subtitle) 

HF Broadband Whip Antenna Evaluation 



5. TYPE OF REPORT ft PERIOD COVERED 

Final - 1 Jul to 1 Oct 1984 



6. PERFORMING ORG. REPORT NUMBER 



7. AUTHORf*; 

Richard W. Adler 
H. B. Shaw III 



8. CONTRACT OR GRANT NUMBERfi) 



9. PERFORMING ORGANIZATION NAME AND ADDRESS 

Naval Postgraduate School 
Monterey, CA 93943 



10. PROGRAM ELEMENT, PROJECT, TASK 
AREA * WORK UNIT NUMBERS 



N0003984WREF467 



II. CONTROLLING OFFICE NAME AND ADDRESS 

Naval Electronic Systems Command 
PDE 110-12 

Machin^nn TiC 70.363 

14. MONITORING AtJENCY NAME 4 ADDRESSf/f different from Controlling Office) 



12. REPORT OATE 

October 1984 



13. NUMBER OF PAGES 

14 



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UNCLASSIFIED 



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16. DISTRIBUTION STATEMENT (of thle Report) 



Approved for public release; distribution unlimited 



17. DISTRIBUTION STATEMENT (of the abetrmct entered In Block 20, If different from Report) 



18. SUPPLEMENTARY NOTES 



19. KEY WORDS (Continue on reveree elde If neceeeary and Identity by block number) 

Antenna evaluation, whip antenna, broadband antenna 



20. ABSTRACT (Continue on reveree elde If neceeeary and Identify by block number) 

Present Navy shipboard antenna couplers used with whip antennas will not 
suffice for future frequency agile transmitting systems. A new broadband whip 
antenna which operates without a coupler should be evaluated for potential 
application in a frequency hopping mode. VSWR for various mounting configura 
tions and gain relative to a 35 foot whip were measured in this initial por- 
tion of the evaluation. VSWR was typically less than 2:1 and relative gain 
averaged -19 dB with respect to a standard Navy 35 foot whip. 



DD ,^3 1473 



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TABLE OF CONTENTS 



Page 



LIST OF FIGURES ii 

SUMMARY ii: 

INTRODUCTION 1 

DESCRIPTION OF ANTENNA UNDER TEST 2 

SCOPE AND LIMITATIONS OF THE MEASUREMENTS 3 

VSWR, RELATIVE GAIN AND POWER HANDLING TEST RESULTS 4 

CONCLUSIONS AND RECOMMENDATIONS 5 



LIST OF FIGURES 

Page 

Fig. 1. VSWR vs. Frequency for 10 x 20 Foot Ground Plane 7 

Fig. 2. VSWR vs. Frequency for 10 Foot Mast Mount 8 

Fig. 3. VSWR vs. Frequency for 20 Foot Mast Mount 9 

Fig. A. VSWR vs. Frequency for Whiplying on Ground Plane 10 

Fig. 5. VSWR vs. Frequency for Various Mountings 11 

Fig. 6. Relative Gain Over 35 Foot Whip/Coupler 12 

Fig. 7. Reverse VSWR for Comparison to Gain 13 



li 



INTRODUCTION 

Marine whip antennas typically operate as monopoles, and when mounted on 
ships, exhibit input impedance characteristics over the HF band (2-32 MHz) 
which vary greatly from the classical 50 ohm load impedance to which most 
transmitters and receivers are designed. Most U.S. Navy whips are 35 feet 
long and are quarter-wave resonant at approximately 7 MHz. Below 7, the whip 
is capacitive and the radiation resistance drops to a value of 2 ohms at the 
lower limit of 2 MHz. Above 7 and up to 14 MHz, typical input impedance 
swishes around the Smith Chart with VSWR values ranging from 1.3:1 to 22:1. 
At 14 MHz, half-wave resonance produces a high resistance and high VSWR. 
Every 7 MHz thereafter will show additional low and high impedance resonances 
with inductive and capacitive reactances in between. 

The usual method for taming the wild impedance excursions and allowing 
reasonably efficient power transfer to communication tranceivers wanting 50 
ohm antennas is to use impedance transforming and reactive matching units 
called antenna couplers. Modern Navy whip couplers are quite efficient with 
less than 1 dB of insertion loss above 2 MHz and worst case loss or -3 dB down 
at 2 MHz. Many of today's couplers are auto-tune units, meaning they auto- 
matically sense reflected power and adjust their internal variable inductors 
and capacitors for minimum VSWR. This process can take as long as 45-60 
seconds and is seldom complete in less than 2 seconds. 

Future Navy frequency-hopping communication systems may jump between 
widely different frequencies at a rate that exceeds one thousand steps per 
second. The present state of coupler technology eliminates standard whip- 
coupler antennas for application with these frequency-hop systems. What is 
needed is a broadband radiator with VSWR less than some reasonable value, 3:1 
typically. Present Navy antenna designs which can achieve this type of 

1 



SUMMARY 
Present Navy shipboard antenna couplers used with whip antennas will not 
suffice for future frequency agile transmitting systems. A new broadband whip 
antenna which operates without a coupler should be evaluated for potential 
application in a frequency hopping mode. VSWR for various mounting config- 
urations and gain relative to a 35 foot whip were measured in this initial 
portion of the evaluation. VSWR was typically less than 2:1 and relative gain 
averaged -19 dB with respect to a standard Navy 35 foot whip. 



iii 



performance are limited to rather large multiple-wire fan arrays. Typically, 
two such fans would be required, operating from 2-7 and 7-22 MHz. For 
destroyer class vessels and larger, this solution to the broadband antenna 
requirement is probably acceptable but for small lightweight, high-speed ves- 
sels such as a patrol hydrofoil missile craft (PHM), the structure required to 
support a set of fan antennas is too tall and too heavy. Only a "lightning- 
fast" coupler-whip combination seems acceptable. 

DESCRIPTION OF ANTENNA UNDER TEST 
Triad Micro-system, Inc. of Santa Ana, CA recently acquired a broadband 
Marine HF whip antenna which was advertised as a "extremely wide-band totally- 
passive HF vertical antenna requiring no ATU (Antenna Tuning Unit or 
Coupler)." Performance indicated VSWR of less than 2:1 from 1.5 to 30 MHz 
with 500 watts of continuous power handling and 2000 watts P.E.P. The whip is 
7 m. (23 ft.) long and weighs only 9 kg., withstands 160 km/h winds and 
"requires no ground plane." The manufacturer also described several inter- 
esting characteristics: 

1. Dielectric loading for more efficiency. (Distributed capacitively 
loaded monopoles have more uniform current distributions, hence more 
radiation. ) 

2. Low-Q radiator with a resistance of 4 times the feed impedance in 
parallel with the antenna. (Base loading resistors will absorb 
reflected energy but also some forward energy.) 

3. Performance is similar to a 7 m whip and ATU combination. (A 
resistively loaded antenna would be expected to exhibit lower gain 
due to losses. ) 



A visual inspection shows fiberglass whip construction and an 18 inch 
long finned heat at the base, which contains the swamping resistors and a 3 
port transformer element. 

SCOPE AND LIMITATIONS OF THE MEASUREMENTS 

A measurement program was undertaken to provide performance comparisons 
to a standard 35 foot stainless steel whip antenna. At NPS, the 35 foot whip 
was mounted on the corner of a 10 foot by 20 foot ground plane, on the 7th 
deck of the Electrical Engineering Building, Spanagal Hall. A micro-processor 
controlled HF transceiver with an automatic tuning unit was connected through 
a directional wattmeter to the 35 foot whip and then to the broadband 7 m 
whip. 

Forward and reverse power measurements provided VSWR for the broadband 
whip. A receiving station was established at a distance of 1.5 km from the 
NPS campus. An EMC-25 field strength meter was used in conjunction with a 45 
degree, 20 foot high sloping vee wire antenna to measure the ground wave sig- 
nal transmitted from both whips for a relative gain comparison. The received 
signal strength was adjusted for the forward transmitter power at each fre- 
quency so that the same reference values applied to both antennas. 

Additional mounting configurations were tested for the broadband whip. 
Mast mounts of 10 feet and 20 feet were available and were used for additional 
VSWR runs. An extreme case of an unfavorable environment, laying the whip 
down flat on the ground plane, was tested to see how effective the base 
matching circuitry was at suppressing high VSWR. 

A power handling test was conducted at a typical VSWR of 1.9:1. Initial- 
ly 500 watts continuous (rated power) was tried, then 750 w. and 1000 w. were 
applied, in an attempt to exceed the capabilities of the antenna. 



^mm wmm 



^^^^^^^^m^^^^^^^^^^*mm 



■^ 



JR, RELATIVE GAIN AND POWER HANDLING TEST RESULTS 
A collect- mi i f f'iscrete frequencies were chosen in the range from 2-30 
MHz. Steps w octed for logarithmic frequency spacing, consistent with 
negligible interference to established services. VSWR runs are plotted In 
Figures 1-5. Figure 1, for the 10x20 foot ground plane, shows an average VSWR 
of 1.91:1 with two perfect match points at 16.25 and 27.5 MHz. The 10 foot 
mast mounting resulted in similar results, shown in Figure 2, with the high 
and low regions of VSWR shifted in frequency from the ground plane case. 
Figure 3 demonstrates a more "natural" mounting condition: a 20 foot mast 
under the 23 foot whip, providing a resonating path for reflected currents, 
which should serve to improve performance. Except at 2 frequencies above 24 
MHz, the performance w.-.r. as advertised, at less than 2:1 VSWR. The 16 and 27 
MHz resonances cf the 10x20 foot ground plane and the 10 foot mast niount were 
not as obvious. The average VSWR here was 1.68:1. 

Figure 4 shows that the base Impedance swamping circuitry is quite effec- 
tive and keeps the VSWR below 2.2:1 over 75% of the frequencies tested. This 
hints at possible low radiation performance because forward power is obviously 
being consumed along with the reflected power. Figure 5 is a composite of the 
first 4 figures and reveals that reasonable VSWR exists for all cases of 
mounting from 2 through 22 MHz. The higher frequencies where the antenna can- 
not be considered as "short" produce VSWR values up to 4:1. 

The relative gain of the broadband whip compared to the 35 foot whip is 
plotted in Figure 6. The average value is -19.7 dB and represents the ability 
of the broadband whip to radiate a ground wave signal with a mounting location 
(10x20 foot ground plane elevated above the surface of the earth) similar to 
that found on a ship. Note the peaks of +2.5 dB and +1.2 dB which occur near 
16 and 28 MHz. These two frequencies were the "best matched" frequencies of 

4 



.^ >.■« .- «•. . ».> .■ . - ^ „•. lV _1 



the VSWR runs of Figure 1. Figure 7 is Figure 1 flipped over so as to allow 
comparison of VSWR and relative radiation efficiency. By overlaying the two 
curves it is clear that low VSWR equates to higher gain and vice versa. Power 
dissipation tests at 500 watts where conducted with a thermometer afixed to 
the center region of the heat sink (the input coaxial connnector is at the top 
of the heat sink) in an ambient environment of approximately 22° C. The 
temperature rose to 62° C within 18 minutes and it stabilized there during the 
rest of the one hour test. At 750 watts delivered to the antenna, the temper- 
ature rose to 70° C and stabilized in 15 minutes. For 1000 watts, the temper- 
ature stopped rising in 13 minutes and reached 82° C. No apparent damage 
resulted from this twice rated power test. The hottest part of the heat sink 
was at the connector point and was well above 100° C. These tests might have 
caused some internal changes, but the measured VSWR remained fixed throughout 
the runs. 

CONCLUSIONS AND RECOMMENDATIONS 
The broadband 7 meter whip tested in 3 different mounting environments 
suggests that mounting on the top of a 7 meter mast enhances the impedance 
match and generally produces VSWR of less than 2:1. A mounting configuration 
similar to those a typical shipboard location would provide (10x20 foot ground 
screen elevated above the surface of the earth) produces VSWR under 3:1 over 
the HF band but with a definite loss in radiation efficiency as compared to a 
standard 35 foot (10 meter) Navy whip/coupler system. The nearly 20 dB aver- 
age loss in radiation might be acceptable for some applications such as short 
haul groundwave operation in the frequency range of 3-8 MHz using typical 
transmit power levels. Other applications should be subjected to evaluation 
via a communications link test, preferably with frequency hopping and in com- 
parison to some standard antenna(s). 

5 



An example of a reasonably inexpensive test would consist of a 
microprocess-controlled HF transceiver, controlled by a micro-computer and 
utilizing an error-checking modulation scheme (such as a packet modem would 
provide). Such an experiment could compare standard whips and fan-like 
antennas by operating slowly enough to permit matching the whips at each 
frequency. It would verify the relative gain reported in this study and could 
identify applications where this type of radiator could be used on small 
combatants. The low efficiency could be overcome by applying higher power 
linear amplifiers, if the received signal strength were adequate. 




























































LEGEND 
WHIP ON 10X20 FT GND PLANE 








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Fig. 1. VSWR vs. Frequency for 10 x 20 Foot Ground Plane 



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Fig. 2. VSWR vs. Frequency for 10 Foot Mast Mount 



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Fig. 4. VSWR vs. Frequency for Whiplying on Ground Plane 



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Fig. 5. VSWR vs. Frequency for Various Mountings 



11 























































LEGEND 
BOTH WHIPS ON 10X20 FT GND PLANE 




















































































































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Fig. 6. Relative Gain Over 35 Foot Whip/Coupler 



12 




UMSA 

Fig. 7. Reverse VSWR for Comparison to Gain 



13 



INITIAL DISTRIBUTION LIST 



No. Copies 



1. Library, Code 0142 2 
Naval Postgraduate School 

Monterey, CA 93943 

2. Defense Technologies Information Center 2 
Cameron Station 

Alexandria, VA 22314 

3. Professor R. W. Adler 3 
Naval Postgraduate School 

Code 62Ab 
Monterey, CA 93943 

4. CDR H. B. Shaw, III 1 
Naval Postgraduate School 

Code 33 

Monterey, CA 93943 

5. Naval Electronic Systems Command 5 
PDE-110-12 (L. Cupston) 

Washington, D.C. 20363 

6. Triad Micro-Systems (T. Kampe) 3 
540 N. Golden Circle Drive #210 

Santa Ana, CA 92705 

7. Naval Ocean Systems Center 1 
Code 822 (I. Olsen) 

San Diego, CA 92152 

8. Naval Ocean Systems Center 1 
Code 822 (J. C. Logan) 

San Diego, CA 92152 

9. Naval Ocean Systems Center 1 
Code 754 (R. Rose) 

San Diego, CA 92152 



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